Ultrafast UV Research Articles

Effective Dual Cation Release in Quasi‐2D Perovskites for Ultrafast UV Light‐Powered Imaging

AbstractRuddlesden‐Popper quasi‐2D perovskites represent robust candidates for optoelectronic applications, achieving a delicate balance between outstanding photoresponse and stability. However, mitigating the internal defects in polycrystalline films remains challenging, and their optoelectronic performances still lag behind that of their 3D counterparts. This work highlights the profound impact of defect passivation at the buried interface and grain boundaries through a dual‐cation‐release strategy. Cations released from the pre‐deposited inorganic iodide buffer layer effectively repair deep‐level defects by inducing low‐dimensional phase reconstruction and interacting with undercoordinated ions. The resulting quasi‐2D perovskite polycrystalline films feature large grain size (>2 µm) and minimum surface roughness, along with alleviated out‐of‐plane residual tensile strain, which is beneficial for inhibiting the initiation and propagation of cracks. The fabricated photodetector demonstrates drastically improved self‐powered photoresponse capability, with maximum responsivity up to 0.41 A W−1 at 430 nm and an ultrafast response speed of 161 ns / 1.91 µs. Moreover, this strategy is compatible with the photolithography‐assisted hydrophobic‐hydrophilic patterning process for fabricating pixelated photodetector arrays, which enables high‐sensitivity imaging. This study presents a feasible defect passivation approach in quasi‐2D perovskites, thereby providing insights into the fabrication of high‐performance optoelectronic devices.

Angle-resolved photoemission spectroscopy (ARPES): probing electronic structure and many-body interactions

Complex phenomenon in quantum materials is a major theme of physics today. As better controlled model systems, a sophisticated understanding of the universality and diversity of these solids may lead to revelations well beyond themselves. Angle-resolved photoemission spectroscopy (ARPES), formulated after Einstein’s photoelectric effect, has been a key tool to uncover the microscopic processes of the electrons that give rise to the rich physics in these solids. Over the last three decades, the improved resolution and carefully matched experiments have been the keys to turn this technique into a leading experimental probe of electronic structures and many-body effects. Drawing upon examples spanning from novel superconductors and topological materials to magnetic and one-dimensional materials, we illustrate ARPES's pivotal role in testing ideas, benchmarking theoretical frameworks, uncovering unexpected phenomena, and elucidating the fingerprints of many-body interactions. Moreover, we demonstrate how the integration of modern ultrafast UV lasers and spin polarimetry has empowered photoemission spectroscopy to capture essential microscopic quantities of electrons—energy, momentum, spin, and temporal dynamics—yielding invaluable insights from a wealth of rich and precise information.

Investigation of AlGaN-Delta–GaN-Based UV Photodiodes in a Metal–Semiconductor–Metal Configuration for Efficient and Fast Solar Blind UV Sensing

Metal–semiconductor–metal (MSM) configured UV photodiodes (PD’s) were designed and fabricated on an AlGaN/GaN–based substrate for efficient and ultrafast UV detection. The purpose was to investigate the feasibility of obtaining efficient and ultrafast temporal response from these devices in the UV given the challenges associated with the formation of Schottky contacts on laterally oriented AlGaN/GaN thin films. Two sets of devices were implemented using Pt and Au as metal contacts with 5-μm finger width, 5-μm finger spacing, and a 50-μm × 50-μm active area. Spectral and voltage bias studies were done to establish the spectral profile and the effect of bias voltage on the responsivity of the detectors at 265 nm. The best vertical MSM PD’s produced 0.6-A/W responsivity under 10-V bias voltage at 265 nm. Peak spectral responsivities were recorded as 1.35 A/W and 1.25 A/W at 240 nm for Pt and Au PD’s, respectively.

Ultrafast and Ultrabroadband UV–vis-NIR Photosensitivity under Reverse and Self-Bias Conditions by n+-ZnO/n-Si Isotype Heterojunction with >1 kHz Bandwidth

A high-performance broadband photodetector has attracted significant attention due to its wide range of applications. We report an n+-ZnO/n-Si isotype heterojunction by depositing ZnO nanorods followed by a ZnO thin film on an n-type undoped Si wafer for detecting a broad wavelength from 300 to 940 nm with a high speed under reverse as well as zero bias conditions. Under 1.5 V reverse and zero bias, the device provides responsivity values up to ∼200 and ∼13 mA/W respectively. Under self-bias, the n+-ZnO/n-Si heterojunction exhibits up to 1.2 × 103 photosensitivity. The n+-ZnO/n-Si heterojunction can detect power as low as 10–12 μW/cm2. A photoresponse up to 2 kHz modulated illuminations has been measured, and the ultrafast n+-ZnO/n-Si heterojunction provides a stable and rapid photoresponse with a response time in the 60–120 μs range. The n+-ZnO/n-Si heterojunction exhibits a −3 dB cutoff frequency of >2000 and 1100 Hz under reverse and zero bias, respectively. Therefore, we unprecedentedly demonstrate an n+-ZnO/n-Si isotype heterojunction as a potential candidate for detecting light in a broad ultraviolet to infrared region with a very high speed and >1 kHz frequency bandwidth as well.

Ultrafast charge migration in the laser induced dynamics of LiH validated by a computation-free isotope effect

Charge migration in LiH molecule and its isotopomers LiD and LiT is examined by quantum dynamics including the non-adiabatic effects of nuclear motions. Upon excitation by an ultrafast UV pulse, mass-independent charge migration occurs between the ground and first excited Σ states. The opposite polarity of these states and a relatively small energy gap between them allow the charge to oscillate at early time of the dynamics during 2 fs after the pulse is over. When the nuclei move, a mass-dependent revival of coherence is computed with a period of 80, 107, 123 fs for LiH, LiD, and LiT, respectively.

Critical Evaluation of the Photovoltaic Performance of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI3 have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI)x(BiI3)y thin films with stoichiometric ratios (x/y) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 °C) spin coating process from AgI/BiI3 solutions in dimethyl sulfoxide. Several critical observations on the basis of ultrafast broadband UV–vis transient absorption spectroscopy and photocurrent spectroscopy were made for these Ag–Bi–I materials, which will have a considerable impact on their photovoltaic performance: Their carrier recombination kinetics were independent of the initial carrier number density over the range of 1.3–6.6 × 1017 cm–3 and well approximated by a monoexponential decay with a rate constant krec in the range of 2.0–4.3 × 108 s–1, which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI)x(BiI3)y compounds, thereby suggesting strong electron–phonon coupling, which will favor charge-carrier localization and nonradiative carrier recombination, in agreement with the virtually absent photoluminescence of these materials. In addition, Fourier-transform step-scan photocurrent spectroscopy (FTPS) on AgBi2I7 provided an Urbach energy of 70 meV and found indications for deep defects (0.6 eV below the band gap), which was also consistent with a trap-mediated recombination mechanism. A combination of all of these effects is likely responsible for the still quite low light-harvesting performance of this class of materials, which has been reported to show photovoltaic conversion efficiencies (PCEs) below 5%. Finally, for “AgI-rich” compounds (3:1, 2:1), we found a substantial separate contribution of carrier loss processes through ultrafast relaxation in AgI domains with time constants of 0.73 and 2.4 ps.

Facile one-step synthesis of graphene oxide/polyhedral oligomeric silsesquioxane composite-coated fiber for high-efficiency solid-phase microextraction of polycyclic musks

Efficient fabrication of excellent fiber coatings for high-performance solid-phase microextraction (SPME) is interesting. Here, a high-efficiency synthesis strategy on graphene oxide/polyhedral oligomeric silsesquioxane (GO/POSS) composite-coated fiber was exploited via an ultra-fast UV polymerization in one step, and applied to excellent SPME of polycyclic musks (PCMs). Using methacryloxy siloxane-grafted GO and POSS-methacryl substituted (POSS-MA) as functional monomers, a facile and direct UV polymerization of multiple reactions including alkenyl radical reaction and thiol-ene click chemistry was fulfilled on thiol-pretreated fiber in only 5 min without tedious process. Characteristics such as morphology, FT-IR, XPS, BET and TG of fiber coating were studied in detail, as well as the SPME performance. Attributing to the rigid stereo conformation of POSS and large conjugate plane of GO-based nanosheet, the significant surface area, high hydrophobicity and intrinsic π-π interactions were adopted in fiber coating. In comparison of commercial SPME fibers, POSS-based or GO-based SPME fibers, a superior extraction performance towards PCMs was achieved with GO/POSS-coated fiber. High-efficiency extraction of PCMs was gained with prominent enrichment factors and the sensitive detection limits of PCMs were of 0.04–0.12 ng/L. In particular, the extraction efficiency was robust and still maintained a high level above 93% for PCMs even after 150 cycle's applications. Good recoveries of PCMs reached 86.6%–102.1% and 86.7%–101.3% in river water and cosmetic samples, respectively. It lights an attractive approach to efficiently fabricating robust GO/POSS-coated fiber for high-performance SPME of PCMs.

Magnesium zinc oxide detectors for fast ultraviolet detection

We explored a Mg-alloyed ZnO material (or MgZnO alloy) on a lattice-matched scandium aluminum magnesium oxide substrate for obtaining most effective photodetectors with highest (∼10 A/W) and fastest (up to 3 GHz) responses and two orders of magnitude UV/VIS rejection ratio. Device operation was related to its material electronic properties (carrier lifetime, diffusivity, and diffusion length) via photoluminescence and light-induced pump–probe techniques. Defect-related response is found suitable for sensitive logarithmic detectors, while exciton-related response is favorable to ultrafast linear UV solar blind detectors. Two-contact planar detector design allows their application in low-cost environmental detection systems.

Improve photo-to-dark current ratio of p-Si/SiO2/n-Ga2O3 heterojunction solar-blind photodetector by inserting SiO2 barrier layer

In this study, the solar-blind ultraviolet photodetectors based on p-Si/Ga2O3 and p-Si/SiO2/n-Ga2O3 heterojunctions were fabricated by metalorganic chemical vapor deposition, respectively. Benefitting from the electron-blocking ability of SiO2 dielectric layer insertion, p-Si/SiO2/n-Ga2O3 photodetectors exhibit a very low dark current of 1.14 × 10−12 A, which is 4 orders of magnitude lower than p-Si/Ga2O3 photodetectors (3.22 × 10−8 A). At the same time, a high photo-to-dark current ratio (1.81 × 105) of p-Si/SiO2/n-Ga2O3 photodetectors was obtained under UV light (λ = 254 nm) at −15 V. Meanwhile, the p-Si/SiO2/n-Ga2O3 devices express better photodetection performance, in which the responsivity and EQE are about two times more than that of p-Si/Ga2O3 photodetectors. Furthermore, the photodetector was found to possess impressive photodetection stabilities. Our results indicate that the p-Si/SiO2/n-Ga2O3 photodetector is an excellent candidate for high-sensitivity, ultrafast response solar-blind UV light detection.

Interband transition in plasmonic titanium nitride and its contribution towards ZnO based pyro-phototronic application

Interband transition generated charge carriers in plasmonic TiN have been utilized for optoelectronic device application by fabricating a highly responsive ultrafast pyro-phototronic self-powered UV photodetector. The excitonic-pyroelectric effect of the non-centrosymmetric wurtzite ZnO crystal combined with the interband transition effect of TiN helps in achieving high performance and ultrafast response of the fabricated photodetector. The maximum photoresponsivity of the device is calculated to be 50 mA/W in self-powered condition, which is one of the best values reported in case of ZnO based self-powered UV photodetectors. Apart from this high response, the device also registers an ultrafast response time of 21 μs which is attributed to the inherent pyroelectric effect of the ZnO crystal. Finite difference time domain simulation also reveals the contribution of interband transition in the device operation. This study thus provides an insight about the unexplored area of interband transition in plasmonic materials and its efficient use in combination with excitonic-pyroelectric effect of the base semiconductor.

All-intracavity fourth harmonic generation in a passively mode locked VECSEL for ultrafast UV emission

We demonstrate all-intracavity fourth harmonic generation in an ultrafast passively mode locked vertical external cavity surface emitting laser (VECSEL). This novel laser cavity design utilizes a high Q Z-cavity resonator to buildup the circulating power of the second harmonic for frequency quadrupling the fundamental 1060 nm to 265 nm. The demonstrated UV output has an average power of 525 μW, pulse width of 3 ps, and a repetition rate of 510 MHz. This results in a peak pulse power of 250–300 mW over a 30 min duration without active stabilization equipment.

Ultrafast Ring-Opening Reaction of 1,3-Cyclohexadiene: Identification of Nonadiabatic Pathway via Doubly Excited State.

The photoinduced ring-opening reaction of 1,3-cyclohexadiene (CHD) to produce 1,3,5-hexatriene (HT) plays an essential role in the photobiological synthesis of vitamin D3 in the skin. This reaction follows the Woodward-Hoffmann rule, and C5-C6 bond rupture via an electronically excited state occurs with conrotatory motion of the end CH2 groups. However, it is noted that the photoexcited S1(π,π*) state of CHD is not electronically correlated with the ground state of HT, and the reaction must proceed via nonadiabatic transitions. In the present study, we have clearly observed the nonadiabatic reaction pathway via the doubly excited state of CHD using ultrafast extreme UV photoelectron spectroscopy. The results indicate that the reaction occurs in only 68 fs and creates product vibrational coherence. Extensive computational simulations support the interpretation of experimental results and provide further insights into the electronic dynamics in this paradigmatic electrocyclic ring-opening reaction.

Pyro-phototronic application in the Au/ZnO interface for the fabrication of a highly responsive ultrafast UV photodetector

We report the fabrication of a highly responsive ultrafast UV photodetector which utilizes combined photovoltaic and photo-induced pyroelectric effect or pyro-phototronic effect. The device architecture has a layered configuration where the photoactive layer is composed of gold nanoparticles (Au NPs) and zinc oxide (ZnO). Inclusion of sputter-deposited Au NPs in the device geometry exploits the UV absorption in Au NPs through interband transition along with exciton generation in ZnO, resulting in enhanced photocurrent generation in the UV region (365 nm). Moreover, the inherent pyroelectric property of the synthesized c axis oriented ZnO helps in boosting the speed of the fabricated device. The maximum photoresponsivity and specific detectivity of the fabricated device are calculated to be 4.68 AW−1 and 8.18 × 1011 Jones respectively in biased condition, thus making it one of the best among its compeer UV photodetectors. Additionally, the device demonstrates an ultrafast response time of 15 μs in the self-powered condition, which is another feather in its cap. So, this study opens up a new pathway for the application of combined carrier generation by interband transition and pyro-phototronic effect for the fabrication of highly responsive ultrafast pyro-phototronic UV detector.

Ultrafast UV AlGaN Metal–Semiconductor–Metal Photodetector With a Response Time Below 25 ps

Aluminum-gallium-nitride photodetectors were successfully fabricated with micrometer-scale metal–semiconductor–metal structures and tested with ultrafast, UV laser pulses. The measurements were done with single-shot oscilloscopes. Pulse-broadening effects caused by the measurement system were systematically evaluated and reduced to resolve the intrinsic response time of the detector. The best-performing devices showed a response time of below 25 ps and dark currents below 20 pA. The devices showed linear response with the bias voltage and the laser energy.

Expeditious UV detection of tungstite (WO3·H2O) and tungsten oxide (WO3) decorated multiwall carbon nanotubes (MWCNT) based photodetector: ultrafast response and recovery time

Tungstite (WO3·H2O) and tungsten oxide (WO3) decorated multi-walled carbon nanotube (MWCNT) based ultra-fast UV photodetectors were fabricated and characterized in this work. Tungstite was synthesized by wet chemical route and tungsten oxide was derived by calcinating tungstite at elevated temperatures (723 K and 1023 K). The crystal structures of tungstite and tungsten oxide were confirmed from X-ray diffraction analysis. The UV photo responsive behavior of the tungstite and tungsten oxide decorated MWCNT UV photodetectors were studied using 365 nm UV LED array in the intensity range 15–83 μW cm−2. The device showed good UV photoresponsivity with high external quantum efficiency (EQE). Tungstite decorated MWCNT UV photodetector exhibited maximum responsivity of 7.4 AW−1 along with EQE of 2550% under the incident UV intensity of 15 μW cm−2. Up to 500 Hz optical chopping frequency, the devices showed remarkable stability along with good repeatability of UV detection. Response and recovery time is recorded as low as 400 μs and 500 μs respectively for tungstite decorated UV photodetectors.

Lighting up the new order.

Irradiating a PbTiO3/SrTiO3 superlattice with ultrafast UV light pulses modifies elastic and electrostatic interactions, resulting in the formation of a stable complex 3D supercrystal.

Tunable Lifetimes of Intramolecular Charge-Separated States in Molecular Donor–Acceptor Dyads

We report ultrafast transient UV–vis absorption and electrochemical spectroscopies on the photoinduced charge separation dynamics in a recently synthesized family of metal-free donor–acceptor syste...

Perovskite-quantum dots interface: Deciphering its ultrafast charge carrier dynamics

Understanding electron and hole (e,h) transport at semiconductor interfaces is paramount to developing efficient optoelectronic devices. Halide perovskite/semiconductor quantum dots (QDs) have emerged as smart hybrid systems with a huge potential for light emission and energy conversion. However, the dynamics of generated e-h pairs are not fully understood. Ultrafast UV–VIS transient absorption and THz spectroscopies have enabled us to unravel the processes of the e-h recombination within a hybrid film of methylammonium lead triiodide (MAPbI3) interacting with different amount of PbS/CdS core/shell QDs. To accurately analyze the complex behavior, we applied a new model for e-h events in this hybrid material. The results obtained with sample having a high concentration of QDs (7.3 mass percentage) indicate: (i) a large population (92%) of the photogenerated charge carriers are affected by QDs presence. The main part of these carriers (85% of the total) in perovskite domain diffuse towards QDs, where they transfer to the interface (electrons) and QD´s valence bands (holes) with rate constants of 1.2 × 1010 s−1 and 4.6 × 1010 s−1, respectively. 7% of these affected charged entities are excitons in the perovskite domain in close vicinity of the interface, and show a recombination rate constant of 3.7 × 1010 s−1. (ii) The carriers not affected by QDs presence (8%) recombine through known perovskite deactivation channels. Lowering the QDs mass percentage to 0.24 causes a decrease of electron and hole effective transfer rate constants, and disappearance of excitons. These results provide clues to improve the performance of perovskite/QD based devices.

Photoelastic properties of zinc-blende AlxGa1−xN in the UV: Picosecond ultrasonic studies

Picosecond ultrasonics was used to study the photoelastic properties of zinc-blende (cubic) $c{\text{-Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ with $x$ around 0.5. The velocities for longitudinal sound in the alloys were measured using ultrafast UV pump-probe experiments with (AlGa)N membranes. Strong Brillouin oscillations were observed in (AlGa)N films attached to GaAs substrates. These oscillations are due to the dynamical interference of the probe beams reflected from the sample surface and interfaces and a picosecond-duration strain pulse propagating in the alloy layer. Optical and elasto-optical parameters including the complex refractive index and the fundamental band gap of the cubic nitride alloys are determined and compared with the values obtained by ellipsometry.

Bi2O3 monolayers from elemental liquid bismuth

Atomically thin, semiconducting transition and post transition metal oxides are emerging as a promising category of materials for high-performance oxide optoelectronic applications. However, the wafer-scale synthesis of crystalline atomically thin samples has been a challenge, particularly for oxides that do not present layered crystal structures. Herein we use a facile, scalable method to synthesise ultrathin bismuth oxide nanosheets using a liquid metal facilitated synthesis approach. Monolayers of α-Bi2O3 featuring sub-nanometre thickness, high crystallinity and large lateral dimensions could be isolated from the liquid bismuth surface. The nanosheets were found to be n-type semiconductors with a direct band gap of ∼3.5 eV and were suited for developing ultra violet (UV) photodetectors. The developed devices featured a high responsivity of ∼400 AW-1 when illuminated with 365 nm UV light and fast response times of ∼70 μs. The developed methods and obtained nanosheets can likely be developed further towards the synthesis of other bismuth based atomically thin chalcogenides that hold promise for electronic, optical and catalytic applications.